Examples of ENSO-Society Interactions
Climate and ENSO Associations with Malaria Incidence in Colombia
Germán Poveda
Postgrado en Aprovechamiento de Recursos Hidráulicos,
Universidad Nacional de Colombia
Medellín, Colombia
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More than five million people live in malaria-prone regions of Colombia.
During 1996, transmission of malaria reached 42 cases per 1,000 inhabitants
in high-risk areas. The province of Chocó (along the Pacific coast)
experienced more than 80,000 cases during 1998 when the population at risk
was of 380,000. Malaria is transmitted by mosquito vectors such Anopheles
albimanus, A. darlingi and A. nuñeztovari, transmitting
Plasmodium falciparum (46.5%) and P. vivax (53.5%). Climate
and its variability constitute important factors to explain the incidence
of malaria in low-land tropical endemic areas of Colombia. During "normal
years", endemic malaria in those regions exhibit a clear-cut "normal" annual
cycle, tightly associated with prevalent climatic conditions, mainly mean
temperature, precipitation, humidity and river discharges In general, during
dry seasons malaria incidence increases concomitant and following periods
of reduced rainfall, and river flows, and augmented average and minimum air
temperature. (Poveda et al., 2001).
ENSO is the main forcing mechanism of Colombia's hydro-climatology at
interannual timescales. Overall, Colombia experience droughts--reduced rainfall,
soil moisture, river discharges, and evaporation--along with increase in
air temperature during El Niño events. The reverse is generally valid
for La Niña. The impact of ENSO occurs earlier and stronger in western
and central Colombia than in the east. Seasonally, the impacts of ENSO
are more pronounced during December-January-February (year +1)1, September-October-November (year 0) and June-July-August (year 0), in
that order. The March-April-May seasons in both years 0 and +1 are the
least affected (Poveda et al., 2001b).
During historical El Niño events (interannual time scale) there
is an intensification in the incidence of malaria in Colombia, as reported
by Poveda and Rojas (1995), and Poveda et al. (2000). See Figure 1.
The "normal" annual cycle of malaria is intensified during El Niño
events, at local, regional and national levels. Figure 2 shows the spatial
distribution of average annual cycle (black bars) and during El Niño
events (red bars), for P. vivax malaria recorded at 16 localities
on the lowlands of the northwestern province of Antioquia. Figure 2 shows
the annual cycle considered from the June (year 0) to May (year +1). Figure
2 confirms that malaria cases intensify during El Niño at local level,
considering a "climate-malaria" year from the second semester of year 0 to
the first semester of year +1. This result is also found in multiple towns
in rural Colombia (not shown), and also in malaria-prone regions nationwide.
These malaria outbreaks are associated with a consistent pattern of hydrological
and climatic anomalies occurring during El Niño in Colombia: increase
in mean temperature, decrease in precipitation, increase in dew point,
and decrease in river discharges, all of them favoring malaria transmission.
There is a coupling between the "normal" annual and the interannual patterns
of malaria incidence in Colombia. Such coupling explains why the effect
appears stronger and more persistent during the second half of El Niño's
year (0), and during the first half of year (+1) (Poveda et al., 2001).
The identified coupling between annual and interannual timescales in the
climate-malaria system shed new light towards understanding the exact linkages
between environmental, entomological and epidemiological factors conductive
to malaria outbreaks, and also imposes the coupling of those timescales
in public health intervention programs in Colombia. We are currently developing
modeling and fieldwork to better understand and explain the observed linkages
between climate, ENSO and malaria incidence in Colombia.
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1Note on notation: "Year 0" refers to the calendar year
(January to December) when an El Niño or La Niña event develops;
"year +1" refers to the calendar year after the year an event develops.
~~~
Links
References
- Poveda, G., and Rojas, W., Impact of El Niño phenomenon
on malaria outbreaks in Colombia (in Spanish). Proceedings XII Colombian
Hydrological Meeting, Colombian Society of Engineers, Bogotá,
647–654. 1996.
- Poveda, G., N. E. Graham, P. R. Epstein, W. Rojas, M. L. Quiñonez,
I. D. Vélez, y W. J. M. Martens, Climate and ENSO Variability Associated
with Vector-Borne Diseases in Colombia. In: Diaz, H.F. and V. Markgraf
(Eds.), El Niño and the Southern Oscillation, Multiscale
Variability and Global and Regional Impacts. Cambridge University Press,
pp. 183-204. 2000.
- Poveda, G., W. Rojas, I. D. Vélez, M. Quiñones,
R. I. Mantilla, D. Ruiz, J. Zuluaga, and G. Rua, Coupling between Annual
and ENSO timescales in the Malaria-Climate association in Colombia. Environmental
Health Perspectives, 109, 489-493. 2001a.
- Poveda, G., A. Jaramillo, M. M. Gil, N. Quiceno, and R. Mantilla.
Seasonality in ENSO Related Precipitation, River Discharges, Soil Moisture,
and Vegetation Index (NDVI) in Colombia. Water Resources Research,
37(8), 2169-2178. 2001b.
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